1. Field of the Invention
The present invention is directed to a cathodic corrosion protection composition and method. In particular, this invention is directed to a curable powder coating composition comprising zinc borate and a method for applying the coating composition, which when applied onto a steel, or other ferrous substrate, provides an anticorrosive coating, effective for improving resistance to cathodic disbondment.
2. Description of the Related Art
One means for preventing corrosion of steel materials, in humid conditions containing electrolytes such as brine and salt solution is cathodic protection. Cathodic protection prevents dissolution of steel by maintaining a steel material as a cathode and inhibiting ionization of iron. However, when the iron portion has a large area, consumption of power and a sacrificial anode increases. Therefore, the steel material is not generally used directly for a cathodic protection, but in most cases, a cathodic protection is effected in combination with an organic coating and/or lining. Through this approach, major proportions of the steel material are protected from corrosion by the organic coating, and defective portions occurring in this organic coating such as scratches and pin-holes, can be supplemented by cathodic protection.
In cathodic protection, metal ions are reduced and become insoluble on the metal surface polarized to the cathode. Therefore, a satisfactory effect can be obtained by effecting anticorrosion by applying power in correspondence with the metal ions which are to be dissolved. The amount of the metal ions to be dissolved is proportional to the surface area of the metal, and corresponds to the surface area of the defective portions in the case of the coated steel material. However, it is extremely difficult to know the exact surface area of the defective portions. For this reason, a cathodic protection is generally applied in excess. However, excessive polarization generates hydroxyl ions due to hydrolysis of water at the cathode, so that scratched portions of the organic coating function as a cathode and are always exposed to an alkaline atmosphere. When such a condition occurs, degradation of the organic coating's adhesion points occurs on any of the interfaces between the steel material and the organic coating and between the organic coatings, particularly at the portions at which alkali resistance is weak, and cathodic disbanding of the organic coating occurs.
As a means for restricting such cathodic disbonding, a method of carrying out a chromate treatment or applying a zinc-rich primer coating of a specific thermosetting epoxide resin has been proposed (see Japanese Unexamined Patent Publication (Kokai) No. 59-222275). However, this technique is not sufficient to satisfy the high-level requirement for the cathodic disbonding resistance in recent years, and does not employ a zinc borate based pigment component in the composition. Japanese Unexamined Patent Publication (Kokai) No. 55-142063 discloses a composition consisting of a polyvinyl butyral resin, a liquid epoxide resin, a borate compound, an epoxy-silane coupling agent and phosphoric acid as a pre-treatment composition for baking type. However, this coating composition is directed to a wash primer for pre-treating a metal and is different from the object of the present invention, and the resins used in this reference do not use a curing agent and are thermoplastic resins.
Methods of restricting such cathodic disbanding are generally known. Unfortunately, many of these techniques are not sufficient to satisfy the high-level requirement for the cathodic disbonding resistance in recent years. A method of providing cathodic protection from corrosion by carrying out the steps of steel pre-treatment, applying a zinc-rich thermosetting epoxide resin based powder coating, and subsequently polarizing the coated steel material as cathode, has been described in European Patent EP 0 588 318 B1, Kaga. However, this technique is limited to coatings with relatively high levels (5–75 wt. %) of zinc compounds, which presents issues of solubility over long periods of time, as well as the increased cost for the zinc borate compound.
Therefore, there is a need for coating compositions, preferably powder coating composition, and methods of application thereof, which provide optimum long term and high temperature and humidity cathodic disbondment protection, at a lower cost.